1. Field of the Invention
This invention relates to the field of polyurethane foams. In particular, it is directed toward opened-cell, flexible polyurethane foams with enhanced load bearing properties.
2. Description of the Prior Art
Flexible polyurethane foams are, by definition, soft and elastically deformable. Generally, the force required for a given degree of deformation is a function of the density of the foam. The lower the density, the lower the load-bearing properties of the foam. See J. H. Saunders and K. C. Frisch, "Polyurethane Chemistry and Technology, Part II Technology," Interscience Publishers (John Wiley), 1964 at pp. 61-63.
It is known that the load-bearing properties of low density (1-3 pcf) flexible polyurethane foams can be enhanced by increasing the cross-link density of the polymer. This is done by reducing the average equivalent weight or increasing the functionality of the polyol. However, increasing the load-bearing properties by this means commonly results in increased gellation rate causing closed cell formation and shrinkage of the foam (Saunders and Frisch, p. 60). Thus, when higher functionality, lower equivalent weight polyols are added to a flexible foam formulation, cell opening agents must also be used to prevent shrinkage.
It is also known that load-bearing properties of flexible polyurethane foams can be enhanced by adding reinforcing fillers to the formulation (Saunders and Frisch, p. 69; Journal of Cellular Plastics, 1981, 43). Such reinforcing fillers can be either inorganic materials (e.g., carbon black, barium sulfate) or organic products such as polyol dispersions of vinyl polymers (U.S. Pat. No. 3,652,639; U.S. Pat. No. 3,383,351), polyhydrazodicarbonamides, (U.S. Pat. No. 3,325,421) or polyisocyanurates (U.S. Pat. No. 4,359,550). These fillers have the common property of providing an insoluble solid phase to the formulation. The resulting foams are both more open-cell and have higher load-bearing properties than foams from the same formulation without added reinforcing fillers. Such two-phase additives, while effective, are relatively more expensive compared to conventional polyols (i.e., those prepared using only alkylene oxide and an initiator). Also, the presence of solids can possibly incur problems during foam preparation due to the separation or coagulation of the solid phase in feed tanks and the plugging of filters and mixing head of the foam machine.